The present invention relates to an improvement of an imaging optical system of a single focus for imaging a photographic subject image in various types of cameras including a so-called silver halide camera, particularly a digital camera, a digital video camera and the like, and particularly, to an imaging optical system in an imaging device using an electronic imaging unit, such as a digital camera or a digital video camera, a camera device using such an imaging optical system, and a mobile information terminal device having such an imaging function.
A market for a so-called digital camera for obtaining digital image data of a photographic subject using a solid-state image sensor has been remarkably large. User's demands for digital cameras of this kind and the like are widely ranged. Among them, users have a high expectation of the category of a high-quality compact camera which uses a relatively large imaging element a diagonal length of which is approximately 20 mm to 45 mm, and on which an imaging lens made up of a high-performance single-focus optical system is installed. The user, among the demands, imposes higher weight on factors of high performance, and in addition thereto, a small F number, that is, being excellent in portability while having a large aperture, that is, a compact size and a light weight.
Here, in terms of high performance, the compact camera is required to have resolving power corresponding to an imaging element of at least 10 mega pixels to 20 mega pixels, in addition thereto, less coma flare, high contrast, and no point image distortion in a peripheral part of a field angle at an open aperture, less chromatic aberration and no occurrence of unnecessary coloring in a part having a large brightness difference, less distortion aberration, and capability of drawing a straight line as a straight line, and so on.
Further, the compact camera is, in terms of a large aperture, required to have an F number which is approximately at least F2.8 from a viewpoint of the necessity of enhancing the differentiation from a general compact camera on which a zoom lens is mounted.
In terms of downsizing, it is required to control both an entire optical length and a diameter of the lens to be small. Further, in terms of downsizing at non-photographing, it is required to have a mechanism called a retractable type in which an air gap on an optical axis in a photographing optical system, such as before and after a diaphragm or a back focal length at non-photographing is shortened to shorten the entire lens length.
Since many users hope that a field angle of a photographing lens has a wide viewing angle to some extent, it is preferable to have a 76-degree field angle equivalent to 28 mm of a focal distance in 35 mm-silver halide camera (so-called Leica size) conversion or a more-degree field angle, that is, 38 or more degrees of a half field angle.
In regard to imaging lenses for digital cameras of this kind, there are considered many kinds of imaging lenses, but an example of the configuration representative of a single focus lens having a wide viewing angle may include a so-called retrofocus type in which a lens group of negative refracting power is disposed on an object side and a lens group of positive refracting power is disposed on an image plane side. The primary reason the retrofocus type is adopted is that there exists a demand for setting an exit pupil position at a distance from an image plane and making peripheral light flux incident at an angle nearly vertical to a sensor, based upon characteristics of an area sensor having a color filter and a microlens for each pixel. However, as understood from a point that the retrofocus type optical system has the purpose of ensuring a back focal length for using a wide-angle lens as an interchangeable lens of a single-lens reflex camera, an entire lens length (distance from a surface on a most object side to an image plane) tends to be long.
On the other hand, in a relatively large imaging element a diagonal length of which is approximately 20 mm to 45 mm, because of improvement or optimization of an on-chip microlens, advancement in image processing or the like in recent years, there occurs no big problem even if the peripheral optical flux is rather obliquely incident to the sensor. Specifically, even if an angle between a main beam and an optical axis at the maximum image height is approximately 35 degrees to 40 degrees, it is becoming possible to establish a sufficiently allowable system. In consequence, it is possible to select a lens type more suitable for downsizing without relying on vertical incidence of the peripheral optical flux as much as before. Under these circumstances, an example of the optical system of the lens type more suitable for downsizing than the retrofocus type optical system may include an approximately symmetrical type optical system or a telephoto type optical system in which a lens group of negative refracting power is disposed on an image plane side. Examples of the conventional imaging lens of such a lens type are disclosed in Japanese Patent Application Publication Numbers H08-313802, H11-326756, 2005-352060, and the like. Further, an example of the lens in which the feature of the retrofocus type remains is disclosed in Japanese Patent Application Publication No. 2012-008347.
The imaging optical system disclosed in Japanese Patent Application Publication No. H08-313802 is an approximately symmetrical type of wide-angle lens, and is provided with a sufficiently large aperture, but there still remains a problem with downsizing since an entire lens length (distance from a surface on a most object side of a lens system to an image plane) or a total lens thickness (distance from the surface on the most object side of the lens system to a surface on a most image side of the lens system) is large. The imaging optical system disclosed in Japanese Patent Application Publication No. H11-326756 also has the configuration close to a symmetrical type where a half field angle exceeds 50 degrees to establish a wide viewing angle, but has a large F number of F4 to F4.5, and has a purpose different from that of the present invention. The imaging optical system disclosed in Japanese Patent Application Publication No. 2005-352060 is configured such that the entire lens length and the total lens thickness are small, but because of a short back focal length, a diameter of a lens close to an image plane is large. Therefore, it cannot be said that this imaging optical system is also sufficiently small-sized. The imaging optical system disclosed in Japanese Patent Application Publication No. 2012-008347 is improved on a point of a field angle, F number and imaging performance, but has the configuration in which the feature of the retrofocus type clearly remains, and still has the problem with downsizing.